Xubuntu 12.04 (Precise) Shortcut Alt-C cannot be assigned in the keybindings dialog because it is also the accelerator for the dialog's Cancel button. The workaround is to edit the preferences file by hand and set the accelerator to "<Alt>c".
Or hover the menu item and press the combo.
Shortcut editor is gone.
x *** This bug has been marked as a duplicate of bug 6440 ***